Fine particles composed of a small amount of iron or iron compounds are generally contained in a petroleum heavy oil. These fine particles come to be contained in the above petroleum heavy oil as fine particles and the like peeled off from tanks, pipelines and a distillation apparatus owing to corrosion of them when a crude oil is transported from an area of production by a tanker, stored in a tank and fed to the distillation apparatus through pipelines. When such a petroleum heavy oil, particularly a petroleum heavy residual oil is used as a feed oil in a fixed bed hydrogenation apparatus (hydrodesulfurization or hydrocracking apparatus), a fine particulate iron component contained in the feed oil deposits on a catalyst or between catalyst particles to clog the reactor thereby increasing pressure drop or to reduce activity of the catalyst particles. Clogging of the reactor increases pressure drop and deflects the feed oil to reduce its flow rate and sometimes to shut down operation of the apparatus. Deterioration of the catalyst is required to replace the catalyst, resulting in a significant loss for the operation of the hydrogenation apparatus.
In general, in petroleum refining industry, a filter for solid materials is usually provided in a raw material oil feeding line in order to remove solid impurities contained in the oil. The conventional filter is to prevent damage of a pump and the like, and it can separate solid materials having a large particle size by filtering, but can not separate to remove fine particles of the order of microns as described later. These fine particles are considered to be fine particles composed of iron compounds as mentioned above and have been become a main factor for obstructing the operation of the hydrogenation apparatus. In order to remove these fine particles, it has been attempted that a filter paper and a membrane filter having a fine mesh are used as a filter or a centrifugal separator is used. The above filter, however, causes great pressure drop and also rapid clogging so,that it cannot be used practically for a long time. Even if the filter part is replaced, the replacement is required to be often conducted so that the filter is unsuitable for treating a large amount of the raw material oil. The centrifugal separator also has a problem with respect to performances and was not of practical use.
Recently, a high gradient magnetic separator has been attempted to be used to remove magnetic particles in a fluid. In the high gradient magnetic separator for removing magnetic fine particles, a ferromagnetic packing is placed in a space of a high magnetic field, and a high magnetic field gradient is caused to form around the packing, thereby magnetic fine particles are caused to adhere to the packing to separate from the fluid. The high gradient magnetic separator has been developed and utilized in the fields such as chemistry, iron and steel, mineral dressing, water treatment and prevention of environmental pollution. Utilization of the high gradient magnetic separator in petroleum refining industry is first attempted in Japanese Patent Laid-Open No. 62-54790 by which it was found that iron fine particles can be removed by the use of the magnetic separator. However, the above patent application was incomplete as a system which can be commercially run.